(1) Field of the Invention
The present invention relates generally to subsurface deployable antennas and, more particularly, to a cable deployed antenna array having watertight antenna elements which move away from each other for operation, and then move toward each other for compact storage.
(2) Description of the Prior Art
It is a challenge for undersea vehicles to maintain broadband communications via satellite links and with surface vessels while remaining undetected at cruising speed and depth.
The electrical conductivity of seawater inhibits electromagnetic radiation (radio signals) from penetrating to significant depths below the sea surface. At HF, VHF, UHF and microwave frequencies, the “skin depth,” or depth to which a given wavelength of radio energy will penetrate, is limited to a few centimeters. As a general rule, penetration into seawater is shallower with decreasing wavelength (increasing frequency). Therefore, without an external antenna, an undersea vehicle cannot remain undetected and communicate at depth and speed.
For communications at cruising speed and depth, undersea vehicles have had to rely upon extremely low frequency (ELF) transmissions. The ELF wavelength of the electromagnetic radiation is measured in kilometers (frequencies in the kilohertz range). Seawater skin depths at these frequencies permit communications to some significant depth, but the long wavelengths limit the bandwidth of the transmitted signal. Consequently data rates are extremely low. This low data rate can make it difficult or impossible to transmit lengthy messages or images.
For undersea vehicles to currently communicate at shorter wavelengths (i.e., HF, VHF, UHF and microwave frequencies) the undersea vehicle can come near to the surface and extend a whip antenna or mast above the sea surface. However, at shallow depths, the undersea vehicle is easily detectable and vulnerable.
Towed buoyant antennas have been developed over time and successfully employed, but not without some technical deficiencies at ELF through HF wavelengths. Research has been continuing into developing low-profile antenna arrays for satellite and surface vessel communications in the VHF, UHF and microwave regimes. At these shorter wavelengths, significant bandwidth is available and consequently higher data rates can be achieved.
However, the relatively small and low profile antennas are susceptible to washover. Washover is the condition where an antenna becomes inundated by seawater. The degree and frequency to which an antenna is briefly rendered inoperable by washover is a function of antenna geometry, speed and course, as well as sea wave parameters.
Current developments in low-profile antenna design employ multiple element antennas to overcome the effects of washover. Multiple antennas in an array increase the probability of at least one antenna being exposed rather than washed over at any given time. Multiple antenna arrays are also used for increasing overall antenna gain and to permit array phasing and directional beam forming. In the prior art, these multiple antenna arrays require substantial storage space when not deployed. The problem is that undersea vehicles have very limited available storage space.
The following United States patents describe various prior art systems that may be related to the above and/or other electronics equipment storage systems:
U.S. Pat. No. 6,870,508, issued Mar. 22, 2005, to Rivera, discloses a slotted antenna with a plurality of loop structures and interconnecting conductors that define a slot. The antennas can operate in a single band or over multiple bands. Flexible or inflatable substrates enable easy storage aboard an underwater craft and facilitate deployment and towing behind an underwater craft with minimal chances of detection.
U.S. Pat. No. 7,226,328, issued Jun. 5, 2007, to Puzella et al., discloses an extendable spar buoy sea-based communication system includes a spar buoy having a retracted configuration deployable from an underwater vessel and an extended configuration after deployment, and a communication subsystem mounted to the top of the spar buoy and supported thereby.
U.S. Pat. No. 7,199,763, issued Apr. 3, 2007, to Bryan, Jr. et al., discloses an antenna system for operation near a ground plane, for example, at or near the surface of a body of water. The system also includes, for example, a buoyant support and/or housing for transporting the antenna to and/or maintaining the antenna at or near the surface of a body of water.
U.S. Pat. No. 6,710,746, issued Mar. 23, 2004, to Anderson et al., discloses an antenna having a reconfigurable length, and a method of reconfiguring an antenna. The antenna can comprise an enclosed composition capable of forming plasma operable as an antenna, an energy source configured for applying variable energy levels to the composition to thereby form variable plasma configurations, and an enclosure containing the composition. The enclosure can have a proximal end, wherein upon application of a first energy level to the composition, a first plasma length with respect to the proximal end is formed, and upon application of a second energy level to the composition, a second plasma length with respect to the proximal end is formed.
U.S. Pat. No. 6,683,579, issued Jan. 27, 2004, to Rivera, discloses a compact antenna that transmits and receives broadband electromagnetic energy signals over a hemispherical pattern in air at the ocean's surface. The antenna has an elongate semi-cylindrical shell, a circular end disc connected to one end, and a semi-circular end disc connected to the other end to form a half-cylinder cavity. A curved plate is connected to the circular end disc and a curved body portion has a curved end extending parallel with the semi-cylindrical shell. A vertical stem has an upper portion connected to the curved plate. The semi-cylindrical shell, circular end disc, semi-circular end disc, and vertical stem are made from a material that is conductive of electromagnetic signals, and they can be differently dimensioned to change the center frequencies to embrace different broadband ranges. The antenna is deployed and retrieved from a submarine in a tow body.
U.S. Pat. No. 6,127,983, issued Oct. 3, 2000, to Rivera, discloses a wideband, low-profile, towable submarine antenna. The antenna is formed with a metal cylinder having a longitudinal slot. The entire antenna may be encapsulated in a tow body and towed horizontally on the surface of the water. The longitudinal slot is open at one end and closed, or shorted, at the opposite end. The location of the antenna feedpoint is placed along the slot so as to set up two sets of frequency resonances. This configuration provides two voltage standing wave ratio minimums, thereby extending the effective reception and transmission range over the entire military UHF frequency range (225-400 MHz).
U.S. Pat. No. 5,977,918, issued Nov. 2, 1999, to Sirmalis, discloses a hydrodynamically and stealth shaped suite of antennas and sensors mounted above a hydrodynamically and stealth shaped mast. The mast is configured to extend from the sail of a submerged vessel to the surface so as to provide the vessel with satellite communications capabilities. The hydrodynamic shape of the mast minimizes the surface wake and the stealth shape of the antenna suite and mast minimizes the radar cross section of the system, lessening detection by hostile forces. To obtain a stealth shape, the radar suite utilizes a conformal phased array antenna. In addition, electro-optical sensors in the antenna suite provide visual, environmental and other sensing capabilities to the system. Further, structurally embedded ESM antennas provide ESM and radar capabilities. When in the retracted position, the antenna system conforms to the hydrodynamic and stealth shape of the submerged vessel and the conformal phased array of the antenna suite forms an integral part of the conformal phased array of the vessel.
U.S. Pat. No. 5,940,046, issued Aug. 17, 1999, to Saleem, discloses an easily reconfigurable mast antenna system disposed within an RF transparent radome where modularly, interchangeably designed antenna of the system are detachably attached to each other via an electronic interface connector and to a carrier backbone. RF, DC and fiber optic cables transmit signals to antennas operating in the range of 5 kHz to 18 GHz.
U.S. Pat. No. 5,933,117, issued Aug. 3, 1999, to Gerhard, discloses a buoyant loop antenna, deployable along a cable, which includes a core region comprising a plurality of annular ferrite beads. These annular shaped beads include a center hole and generally concave first end and a generally convex second end. The ferrite beads are aligned with the concave end of one bead against the convex end of another bead. This allows the cable to flex while the beads maintain contact with each other, providing flexibility and resistance to crushing. The core region has a loop wire wrapped helically around it, forming the loop antenna. The loop wire element starts and ends at the same end of the core region, forming a loop. This loop allows transmission and reception in and athwart (side to side) direction. This novel wire loop antenna can be combined with a straight wire antenna (which provides reception in a fore and aft direction) to provide an omni-directional cable antenna assembly.
U.S. Pat. No. 5,319,376, issued Jun. 7, 1994, to Eniger, discloses an improved ice penetrating arctic buoy for release from a submarine submerged under a layer of ice. The device includes a Global Positioning System navigational processor receiver and/or a Emergency Position Information Radio Beacon and/or surveillance apparatus, an antenna for transmission and/or reception of appropriate RF frequencies, such as Global Positioning System or other R.F. frequencies found in the environment above the ice or EPIRB R.F. frequencies originating in the buoy into the environment above the ice. Convertor means converts navigational information and/or other information outputted from said Global Positioning System receiver into coded corresponding information. An onboard transmitter sends coded information acoustically or optically and/or electrically to the submarine via an underwater data link.
U.S. Pat. No. 7,165,504, issued Jan. 23, 2007, to Miller, discloses an antenna linear extension and retraction apparatus and method of use for a submersible device. The apparatus includes a body having a cavity formed at an external surface. A bladder containing a core material is arranged within the cavity. The core material contracts and expands the bladder depending upon a pressure that surrounds the bladder. An antenna is operatively connected with the bladder and moves between a retracted position and a deployed position as the bladder contracts and expands.
The above-cited prior art does not adequately disclose a low-profile, undersea antenna array implementation that is stored in a retracted (or physically compressed) state while stowed in a limited storage space, but upon deployment, expands to a full operational dimension. As such, a continuing need exists for an antenna that reassumes a compact state during the retrieval and stowing process. Such a novel implementation would allow for control over the inter-antenna element spacing, thus making it feasible to mechanically adjust the antenna for optimum hydrodynamic performance (and minimal washover) for a given sea state. The same ability to modify antenna element spacing would be applicable to array phasing and beam forming considerations. Summarizing, a long felt need exists for the solutions to the above-described and/or related problems.